The present invention relates to a zoom lens and an image-pickup apparatus, and more particularly to an image-pickup apparatus, such as a video camera, an electronic still camera, a broadcasting camera, or a monitoring camera, which uses a solid-state image-pickup element.
A high function has been achieved for the image-pickup apparatus, such as a video camera, a digital still camera, a broadcasting camera, a monitoring camera, or a silver-haloid film, which uses the solid-state image-pickup element to miniaturize the entire apparatus.
The image-pickup apparatus requires, as an image-pickup optical system to be used, a zoom lens short in total length and compact, and having a high zoom ratio (large magnification) and a high resolving power.
To meet such a requirement, a zoom lens has been developed which includes, in order from an object side to an image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a rear unit including at least one lens unit disposed further on the image side than the second lens unit (Japanese Patent Laid-Open No. [U.S. Pat. No. 5,0963,378], Japanese Patent Laid-Open No. 2000-347102 [U.S. Pat. No. 6,867,925], Japanese Patent Laid-Open No. 2003-315676 [U.S. Pat. No. 6,975,461], and Japanese Patent Laid-Open No. 6-148523).
Lens configurations of the rear unit have been known which include a configuration formed by one lens unit of a positive refraction power, a configuration formed by plural lens units of positive refractive powers, and a configuration formed by at least one lens unit of a positive refractive power and one lens unit of a negative refractive power.
The zoom lenses of such types perform zooming by moving the second lens unit and after to enlarge or reduce an image formed by the first lens unit.
A zoom lens of a positive-lead type in which a first lens unit has a positive refractive power enables easier acquisition of a higher zoom ratio as compared with a negative-lead type in which a first lens unit has a negative refractive power.
Transparent (transmissive) ceramics has recently been developed, resulting in an image-pickup optical system which uses the transparent ceramics as an optical material. The transparent ceramics has a refractive index, hardness, and strength higher than those of optical glasses. An image-pickup apparatus that uses these characteristics to thin the entire lens system has been developed (Japanese Patent Laid-Open No. 2006-84887 [U.S. Publication No. 2006/0062569]).
The zoom lens used for the image-pickup apparatus is strongly required to be high in zoom ratio and compact in entire lens system.
Generally, to miniaturize the zoom lens, the number of lenses only has to be reduced while a refractive power of each lens unit constituting the zoom lens is increased.
However, once the zoom lens is configured this way, a lens thickness increases to make inadequate a reduction effect of the lens system and simultaneously difficult correction of various aberrations.
Thus, to achieve a high zoom ratio and compactness of the entire lens system, a zoom type, a refractive power of each lens unit, and a configuration of each lens unit have to be appropriately set.
A material used for the lens has to be selected, in view of a refractive index and an Abbe number, such that various aberrations including a chromatic aberration can be reduced at each zoom position.
For example, to achieve a high zoom ratio while miniaturizing the entire zoom lens system of the positive-lead type, an appropriate material has to be selected for a lens element(s) in the first lens unit in view of the refractive index and Abbe number.
The image-pickup apparatus disclosed in Japanese Patent Laid-Open No. 2006-84887 [U.S. Publication No. 2006/0062569]) uses the transparent ceramic for a material of a negative lens element in a cemented lens element in which a positive lens element and the negative lens element are cemented to reduce a lens thickness, thereby miniaturizing the entire lens system.
The image-pickup apparatus disclosed in Japanese Patent Laid-Open No. 2006-84887 [U.S. Publication No. 2006/0062569] is designed based on recognition that ceramics has a refractive index, hardness, and flexural strength higher than those of optical glasses, which enables miniaturization by an amount equal to one thinned negative lens.
In the optical glasses, generally, when the refractive indexes and Abbe numbers thereof are mapped on a graph (hereinafter referred to as “nd-νd diagram”) respectively to be larger in value upward on a vertical axis and larger in value leftward on a horizontal axis, the refractive indexes and the Abbe numbers are distributed almost along several straight lines.
Generally, the optical glasses have characteristics in which a larger refractive index provides a smaller Abbe number, resulting in increase of dispersion.
On the other hand, some ceramics of high transmittance in a visible light wavelength region and some oxide monocrystals or polycrystals are present in a region where the relationship between the refractive index and the Abbe number is different from that of a normal optical glass in the nd-νd diagram.
In other words, materials have been known which have refractive indexes higher than that of the optical glass having an Abbe number equal to the materials.
Use of materials having such a relationship between the refractive index and the Abbe number different from that of the normal optical glass, such as ceramics and oxide monocrystals or polycrystals, is advantageous for aberration correction and miniaturization of the entire optical system.
However, simple use of lens elements made of such materials in zoom lenses does not warrant easy acquisition of high optical performance in the entire zoom range.
To achieve a high zoom ratio and miniaturization of the entire lens system through use of the lens elements made of the above materials, a zoom type, a refractive power of each lens unit, and a configuration of each lens unit have to be appropriately set.